1. Field of the Invention
The present invention relates to a multistage water purification apparatus and, more particularly, to an apparatus for purifying water by means of a plurality of water purifiers arranged in stages corresponding to different depths where the water has different temperatures.
2. Description of the Prior Art
Various methods and apparatus have been proposed hitherto for purifying water in dams, lakes and marshes, rivers and so forth. These known methods and apparatus, however, can not effectively stir and mix the water in the surface region and the water in the deep region independently and, hence, cannot independently make the water temperature and the dissolved oxygen content in these regions uniform. This does not matter substantially when the difference of temperature between the water in the surface region and the water in the deep region is small. However, when the temperature difference is large, e.g. 10.degree. C., the mean water temperature of the water after the stirring comes down below a predetermined level. In the case of a multipurpose dam intended for supplying water for agricultural use, the reduction of water temperature down below a predetermined temperature may adversely affect the agricultural products.